Electrodeposition bath composition and replenishment composition therefor

ABSTRACT

THIS APPLICATION DISCLOSES A COATING BATH COMPOSITION, A HIGH SOLIDS CONTENT REPLENISHMENT COMPOSITION FOR DISPERSION IN SAID BATH AND A METHOD FOR PREPARING A HIGH SOLIDS REPLENISHMENT COMPOSITION FOR DISPERSION IN SAID BATH. THE COATING BATH COMPRISES AN INTIMATE MIXTURE OF WATER, BINDER RESIN AND WATER SOLUBLE AMINO COMPOUND WHEREIN THE WEIGHT RATIO OF BINDER RESIN TO PIGMENT IS AT LEAST ABOUT 2:1 AND SUBSTANTIALLY ABOVE THE SAME RATIO IN THE PAINT FILM THAT WILL ELECTRODEPOSIT FROM SUCH BATH. IN THE REPLENISHMENT COMPOSITION, THIS RATIO IS AT LEAST ABOUT 1.5:1 AND SUBSTANTIALLY THE SAME AS THE SAME RATIO IN THE PAINT FILM THAT WILL ELECTRODEPOSIT FROM SAID BATH.

E. GILCHRIST INVENTOR 3- WW ATI'ORNEYS ALLA/V E. G/LCHR/ST COMPOSITION THEREFOR Original Filed Jan. 4, 1966 ELECTRODE-POSITION BATH COMPOSITION AND REPLENISHMENT April 20, 1971 United States Patent U.S. Cl. 26029.2 7 Claims ABSTRACT OF THE DISCLOSURE This application discloses a coating bath composition, a high solids content replenishment composition for dispersion in said bath and a method for preparing a high solids replenishment composition for dispersion in said bath. The coating bath comprises an intimate mixture of water, binder resin and water soluble amino compound wherein the weight ratio of binder resin to pigment is at least about 2:1 and substantially above the same ratio in the paint film that will electrodeposit from such bath. In the replenishment composition, this ratio is at least about :1 and substantially the same as the same ratio in the paint film that will electrodeposit from said bath.

This application is a division of my copending US. patent application Ser. No. 518,667 filed Jan. 4, 1966, now US. Patent 3,362,899. Application Ser. No. 518,667 is a continuation-in-part of my copending US. Pat. application Ser. No. 375,633 filed June 16, 1964, and now abandoned. Application Ser. No. 375,633 is in turn a continuation-in-part of my copending US. patent application Ser. No. 304,297, filed Aug. 14, 1963,. now US. Patent 3,230,162; and my copending US. patent application Ser. No. 273,270 filed Apr. 16, 1963, and now abandoned. Application Ser. No. 304,297 is in turn a continuation-in-part of my now abandoned U.S. patent application Ser. No. 249,812 filed Jan. 7, 1963, which was a continuation-in-part of my copending US. patent application Ser. No. 186,320 filed Apr. 10, 1962, and my now abandoned US. patent applications Ser. Nos. 132,303 filed Aug. 18, 1961; 183,024 filed Mar. 28, 1962; 186,496 filed Apr. 10, 1962; 202,691 filed June 15, 1962; and 218,575 filed Aug. 22, 1962. Application Ser. No. 273,270 is in turn a continuation-in-part of my now abandoned US. patent application Ser. No. 249,812 and my copending US. patent application Ser. No. 258,913 filed Feb. 15, 1963. Application Ser. No. 258,913 is a continuationin-part of my aforementioned US. patent applications Ser. Nos. 249,812; 186,320; 132,303; 183,024; 186,496; 202,691; and 218,575. The disclosures of these applications are incorporated herein by reference.

This invention relates to a method for painting an electrically conductive object by electrically induced codeposition of a film-forming organic resin having free or dissociable carboxylic acid groups in its molecular structure and pigmentation consisting of at least one component particulate pigment from an aqueous bath and to paint for use in such method. In particular, this invention is concerned with continuous electropainting from an aqueous bath with a multicomponent paint which includes a film-forming polycarboxylic acid resin and a finely divided pigment that remains particulate after electrodeposition and subsequent curing of the resultant film. More particularly, this invention is concerned with con trol of binder-to-pigment ratio in an electropainting bath, the resultant coating and the replenishment feed to said bath.

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This method of coating is carried out at a voltage above the threshold deposition voltage of the paint employed, i.e. the voltage at which deposition of the dispersed binder and pigment in the bath is initiated upon an electrically conductive workpiece when a direct electric current is passed through the bath between the workpiece and a second electrode that is electrically negative in relation to the workpiece, spaced apart from the workpiece, and in electrical contact with said bath. The maximum tolerable voltage is slightly below the rupture voltage of the paint employed, i.e. that voltage at which a paint film already laid down by this method ruptures upon continued application of such voltage during the immersion of the workpiece for coating. With painting compositions now available, thus voltage may be as low as about 20 volts but is advantageously between about 50 and about 500 volts, more commonly about to about 300 volts.

Advantages of this invention include improved compositional stability of the aqueous painting bath and the deposition of superior paint film therefrom characterized by uniformity of pigment-binder distribution.

The painting composition for use in this invention comprises a fluent, intimate, mixture of water, at least one component pigment, film-forming ionic binder resin for said pigment, at least the major fraction of said binder resin being synthetic polycarboxylic acid resin, and sufficient water soluble amino compound to maintain said polycarboxylic acid resin as a dispersion of anionic polyelectrolyte in said composition, the weight of film-forming ionic binder resin to said component pigment in said composition being at least about 2:1 and substantially above the corresponding ratio desired in said paint film.

The preferred polycarboxylic acid binder resins have an electrical equivalent weight between about 1,000 and about 20,000 and an acid number between about 30 and about 300.

A component pigment in the sense used here is one which is infusible at the anode curing temperature used subsequently to electrocoating a paint film thereon and thus remains as discrete solid particles in the resulting cured film. Curing is performed to obtain a tack-free paint surface, and customarily is done by baking in air at a temperature up to about 260 0, although room temperature air curing is useful in some cases, and spraying, dipping, rolling, or otherwise mechanically applying extraneous curing agents to the electrodeposited film also can be useful to yield a tack-free finish on occasion. The electrodeposited film can be cured by itself, or, while still tacky, by spraying or otherwise mechanically applying a. topcoat enamel or similar surface coating in some cases, and the two thus-applied coats force cured together to constitute subsequent curing for the purposes of this invention. The film-forming paint binder resin in my electrocoating operation coalesces on the anode immersed in the bath during the electrocoating operation to hold the discrete paint solids onto the anode so that the anode can be removed from the painting bath and further handled. The term paint as used herein is a composition comprising film-forming binder and at least one component pigment.

In some of my painting compositions, the pigmentation can be with a single component pigment, e.g. a pigmentary grade of titanium dioxide, but more generally the overall pigmentation is made from a plurality of component pigments to achieve various special properties, colors, economies, sag resistance, etc. My electrocoating bath composition must have a binder resin-to-pigment ratio of at least about 2:1 weight to yield a film which does not appear to be dried out and chalky and thus have impaired strength and corrosion resistance. Advantageously, for eificiency and obtaining the best quality electrodeposition, the binder resin-to-pigment ratio in the bath composition is higher, e.g. about 2.5 :1. I have used successfully binder resin-to-pigment ratios as high as 50:1 and even higher, and the ratio can be as high as about 100:1 and higher and still be in a compositional range wherein pigment effects are consequential and important, e.g., for color, resistance to film degradation, apparent bodying, corrosion resistance, etc.

Especially pertinent in this regard is my finding that component pigment solids sustained in my electrocoating hath migrate to the anode more rapidly in the electrocoating operation than does my binder resin when both are considered in bulk, the resin having roughly 70-80% and typically about 75% of the apparent velocity of the pigment. This is shown by a significantly lower weight ratio of film-forming binder resin-to-pigment in the resulting film electrodeposited on the anode than the film-forming binder resin-to-pigment weight ratio of the bath suspension. The ratio in the deposited film is broadly dependent upon and appreciably lower than the concentration of binder resin-to-pigment in the operating bath from which it is deposited. Because of this disproportionate deposition of resin and pigment, establishment and maintenance of a proportion of binder resin relative to pigment in the bath above that desired in the electrodeposited film is essential to obtain consistent pigmented film quality, and because the disproportionation is in the direction of dried-out, chalky film formation, the maintenance of the ratio becomes quite critical when the lower limiting binder resin-to-pigment ratio is approached to obtain especially durable films from the resin and pigment selected for a standardized operating condition. This is contrary to my experience with conventional pigmented painting compositions wherein weight ratios of binder resin-to-pigment often can be below 1:1 with good film formation and there is no disproportionation factor to be considered.

In my painting bath composition, the film-forming binder resin-to-pigment ratio generally is between about 1.25 and 1.4 times that of the film desired to be deposited because of the disproportionation effect. As the kind of binder resin and pigment used and the operating conditions of temperature and voltage can be varied substantially from one particular electrocoating operation to another, it is particularly advantageous to predetermine, e.g. in a small scale trial, a particular resin-to-pigment ratio for the hath, not lower than about 2: 1, necessary for electrodepositing a film therefrom having a desired resin-topigment ratio under the particular electrocoating conditions selected.

In practice, I generally replenish the bath gradually, that is, continuously or incrementally, with a replenishment composition comprising said binder resin and pigmented proportioned to maintain the resin-to-pigment ratio of the bath approximately at the elevated predetermined value, the total replenishment being substantially equal over a sustained operating period to the amount of the binder resin and pigment deposited on the anodes coated. The high solids content replenishment composition for dispersion into my painting composition comprises the intimate mixture of the aforesaid pigment and binder resin in a resin-to-pigment ratio of at least about 1.5 +-O.l):1, this ratio being the approximate lower limit for maintaining the electrocoating bath in sustained operation in a manner to preclude the electrodeposition of chalky, binder-starved appearing films. Advantageously, the ratio is even higher for insuring good film quality conveniently with control tolerance, e.g. at least about 2:1.

The component pigments of my compositions, and by pigment I mean to include for my purposes solid phase fillers as well, are infusible upon subsequent curing. Curing temperature is subject to selection between about room temperature and about 260 C., depending upon whether air curing is adequate or forced curing is necessary or desirable. Under forced curing, some organic particles that are electrodeposited in solid state can melt, coalesce, and smo h out; thus, they are not pigmentary for purposes of that operation, but the same particles are pigmentary in a similar operation where the curing is done at a temperature when they are infusible and remain as discrete particles after curing.

Generally the component pigments are mineral in nature, for example, titanium dioxide, zinc oxide, leaded zinc oxide, white lead, titanium calcium (a composition of titanium dioxide and calcium sulfate), clays such as kaolinite, silica, talc, mica, Wollastonite, calcium carbonate, barium sulfate, ferrite yellow oxide, ferric oxide, brown iron oxide (which is a blend of red, yellow and black iron oxides), tan oxides of iron (which is a similar blend), ultramarine blue, raw sienna, burnt sienna, raw and burnt amber, chromium oxide green, graphite, carbon black, zinc chromate, lampblaok, calcium phosphate, calcium lead chromate, calcium phytate chrome orange, cadmium reds and yellows, alumina, and elemental metal powders such as zinc, aluminum, magnesium and copper. It is advantageous that Water soluble salts in the pigments, such as sodium or potassium salts, are very low and preferably absent so that the electrical resistance of the bath and the film deposition is not sub ject to uncontrolled variation, and that the primary control of electrical conductance is by means of the binder resin and amino compound dispersed in the bath.

The component pigments can also be organic such as the green iron salt of nitroso beta naphthol, copper phthalonitrile blue, toluidine red, parachlor red, para toner (red), alkali resistant red, BON red and maroon, Wachtung red, madder lake red, Duratone red, carmine red, and Hansa yellows. Hard resin particles that are not structurally ionic polyelectrolytes in the painting bath and which remain as discrete solid particles and do not fuse 'with each other in the freshly electrically deposited film at curing temperature are considered broadly as component pigments for my purpose. Typically the solubility of such hard resins in aqueous alkaline dispersion is less than about 1% and they have low acid value for package stability.

Some hard resin particles which are not structurally ionic and not film-forming upon electrodeposition are later fusible at curing temperature, e.g. upon baking at 120-260 C. and various types of such resins can even react with the electrodeposited polycarboxylic acid binder suitably on forced curing for imparting special properties to the resulting cured film such as fire retardance or durability. Examples of such readily fusible resins include dicyclodiepoxy resins, polyethylene resins, polyvinyl chloride resins, certain phenol formaldehyde resins such as those modified with rosin, powdered chlorinated hydrocarbon waxes, epichlorohydrin-bisphenol type epoxy resins advantageously having molecular weight about 1,000 or higher to resist hydrolysis in the bath, chlorinated rubber, vinyl acetate-vinyl chloride copolymers, and mixtures of same that melt and coalesce at the forced curing temperature. Such resin particles deposit on the anode in the same disproportionate way as do mineral pigments and infusible resin particles, but are only considered component pigments for purposes of this invention when they remain particulate after the curing subsequent to electrodeposition and, therefore, do not have any binding effect on the film. If, however, they are fused into a film subsequent to electrodeposition, their particulate character disappears; in such service they are neither film-forming binder resins nor component pigments for purposes of this invention and for computing the critical resin-topigment ratio because they do not coalesce in the electrodeposition nor do they have permanent infusible particulate eifect. Instead, they are considered here as supplementary resinous filming adjuncts independently of that ratio, which adjuncts can be utilized in electrocoating bath dispersions for special effects. Ordinarily they can be used in a proportion up to about 40 and for efficiency preferably not more than 20-25 parts per parts of the synthetic polycarboxylic acid resin electrocoating binder without substantially detracting from electrodeposition of a good film. For purposes of suspension in the bath and even distribution in the resulting electrodeposited film, the particle size of such fusible resins should be very small, preferably no larger than microns average with no more than about 5% exceeding microns.

Because the film-forming polycarboxylic acid binder resin has the attributes of a solute in the bath as distinguished from a latex, the bath viscosity builds up substantially with increasing resin concentration. The small particle size pigments also tend to thicken the bath. For good handling properties, the bath is limited to a maximum concentration of 35% by weight of painting solids (N.V.M.), advantageously to about and preferably, for best all-round painting performance, about 5-12%. Respecting the other limit, extremely low concentration of polycarboxylic acid binder resin give very poor electrodeposition with respect to evenness, smoothness, and adhesion. Accordingly, the lower limit of painting solids for good performance is limited to about 2% by Weight in the bath. To achieve low bath viscosity for purposes of good heat transfer and prevention of too much paint from dragging along when the coated anode is withdrawn from the bath, the bath can contain broadly between about 65% and 98% water and advantageously about 85-98% Water.

Additionally, I have found that very small particle size of all of the component pigments is essential. This is quite in contrast with conventional painting formulations which frequently incorporate component pigment solids of average size well above this with quite good success, for example, component pigments having about 10-30 microns average, and up to about 44 microns. Preferably, for 'best performance, the average particle size of each such infusible pigment component should not be substantially greater than about 2 microns for my purposes. The major reasons for limiting such pigmentation strictly to such critically fine sizes are these: the comparatively high viscosity of a .freshly-electrodeposited resin which has electrical equivalent weight of at least about 1,000 substantially restricts diffusion, mobility or leveling of the codeposited pigment in the electrodeposited film even when it is further cured at an elevated temperature in the range of 120 to 200 C. or higher, the critically small particle size pigment being distributed much more uniformly than coarser material from top to bottom in such film; the high electrical resistance that develops in such resin upon electrodeposition restricts practical film thickness to a comparatively low value so that coarser pigment fractions present often give undesirable roughness and/ or apparent nonuniformity across an expanse of deposited film; and I have found that there is some resin binder enrichment at the base of the film in contact with the anode which enrichment is especially pronounced when coarser pigments are present and not particularly significant when using the critically smaller sized pigments. Where homogeneity of the film is particularly important, I prefer that not only the infusible component pigments, but all particulate materials in the bath are within the critical size limitation.

Furthermore, in view of the necessary comparatively high binder resin-to-pigment ratio required in the film, the critically small pigments can in some cases improve durability of the film, whereas coarser pigments of the same kind usually will not. The pigment sizes I describe here are for equivalent spherical diameters. In addition to being essential for superior film in this type of electrocoating, the small size pigment provides for easy mechanical suspension during the coating operation and easy redispersion after settling from a shut down period.

When a component pigment is fairly dense, e.g. above about 3.5 relative to water, use of a low average particle size of such component pigment not substantially in excess of about 2 microns is particularly advantageous for my purposes; when the component pigment has a lower density than this, the average particle size limit can approach about 5 microns without substantial sacrifice in performance. When infusible pigments are available that have substantially no particles larger than about 10 microns, the use of such pigments is preferred for electrodepositing the best sort of film. However, as many useful pigment components are comminuted from larger particles and then subject to imperfect or incomplete classification procedures, if any, it is usually not economical or practical to remove or avoid minute amounts of oversize material. Thus, as much as about 5% and preferably not more than about 2% by Weight of an infusible component pigment can be oversize, that is, above 10 microns, without substantial sacrifice in performance, but virtually nothing should be coarser than about 44 microns for good suspension in the bath or detraction from good film appearance.

Temperature of the bath body normally is between about 15 and about 50 C., occasionally somewhat warmer, in an extended electrocoating operation. Preferably, however, the temperature for best film deposition is in the range of 20-35 C., and maintenance of bath temperature in this range is thus preferred. The bath is agitated to assist suspension of insoluble solids, to help disperse high solids content additions, and to maintain substantially uniform temperature of the anode surfaces.

The preferred polycarboxlic acid resin for making at least the major fraction of film-forming binder resin for this invention are those disclosed in my copending U.S. patent application Ser. No. 304,297, and have electrical equivalent weight broadly between about 1,000 and about 20,000, the electrical equivalent weight being measured as defined in that copending application. Advantageously, the polycarboxylic acid resin comprises a coupled siccative glyceride oil reacted under substantially anhydrous condltions with about 2-25 of a polymerizable vinyl monomer. In some cases, vinyl polymers such as dicyclopentadiene resins can be used to replace part or all of the vinyl monomer to raise the electrical equivalent weight of the acid resin. Preferably, the vinyl monomer is vinyl toluene limited to about 15% maximum on the weight of the coupled oil, and this resin is extended with non-heat reactive phenol aldehyde resin at an elevated temperature sufficiently long to achieve ostensible chemical union between the polycarboxylic acid resin and this extender so that there is no appreciable dissociation of acid resin and extender in a sustained electrocoating operation, and the product can be considered as a compound rather than a mixture of two compounds. Other extender resins and film plasticizers which coalesce on the anode during the electrodeposition to become effectively part of the paint film binder at that stage can be used as a minor fraction of the film-forming binder. The concentration of such materials should be limited so as not to mask the unique electrical migration characteristics and structural anionic polyelectrolyte behavior of the principal and essential acid resin binder. For best electrodeposition performance, my film-forming binder consists esentially of synthetic polycarboxylic acid resin of the controlled high electrical equivalent weight. The useful binder compositions are film-forming at the electrodeposition bath temperature and are curable to a tack-free film, as for example, by

baking at a temperature of l20-260 C. for about 2-20 minutes. Before curing, they hbld solid phase substances tenaciously to the anode. These features are discussed at length in my copending application Ser. No. 304,297.

Other suitable synthetic polycarboxylic acid resins for my binders include, for example, alkyd resins and acrylic acid and lower alkyl (C substituted acrylic acid-containing polymers, i.e. those having carboxyl groups contributed by alpha, beta carboxylic acids or residues of these acids, having a comparatively high electrical equivalent weight. The comparatively high electrical equivalent weight of the useful synthetic polycarboxylic acid resins for my purpose causes them to convert from dispersed current-carrying anions in the bath to a water resistant and electrically resistant material upon electrodeposition. Their electrical equivalent weight should not be above 20,000 for ease of dispersion and efficiency of operation. The preferred synthetic polycarboxylic acid resins for my purpose have electrical equivalent weight between about 1,000 and 2,000 for deposition of films approaching about 25-50 microns in practical short operating times, e,g. 1-3 minutes. Where a thinner film is desired, the electrical equivalent weight advantageously can be as high as about 5,000.

In addition to use of resinous extenders and film plasticizers of conventional nature, the binder resin can have mixed with it between about 0.1 and about 10%, basis weight of the polycarboxylic acid resin in the bath, of a nonionic organic liquid compatible with the resin in the bath dispersion as a solubilizing assistant or, more accurately, dispersion modifier as shown in my copending application Ser. Nov 304,297. Preferably, these volatilize from the electro-deposited film on forced curing. The proportion of such nonionic organic liquid assistant should be insufiicient to form a distinct phase in the bath composition. Typical assistants are: petroleum naphthas, specifically aliphatic naphthenic and aromatic hydrocarbons or mixtures of same having boiling point between about 30 C. and about 240 C. and preferably between about 150 and 200 C. so that they will volatilize from the film on baking or other curing such as air curing; pine oil, glycerine, water soluble alkoxy alkanols such as 2- butoxy-ethanol-l and other ethylene oxide derivatives of this type, and monoalkyl ethers of glycols such as the monobutyl ether of diethylene glycol. Usually 0.1-6% basis polycarboxylic acid binder resin in the bath is adequate for all purposes.

Additionally, I can use about 12.0% if desired, and most suitably not more than about l5%, basis Weight of the polycarboxylic acid binder resin, of a polybasic carboxylic acid of molecular weight below 1,000 for an amine scavenger. Advantageously, such scavenging acids are polymers of polyenoic acid having 12-44 carbon atoms, preferably the so-called dimer acids which are predominantly the dimer of C unsaturated fatty acids, principally linoleic with some trimer present. These form soaps readily with polyamines at temperatures below about 75 C. and upon codeposition with the film and curing can provide a polyamide material in the resulting film. Those of elevated boiling point such as the dimer acids also can help to plasticize the film and to assist in giving good bond to a subsequent paint coating applied by dip, spray, brush, or roll when the electrically deposited nndercoat has been subjected to overbakingexposure to a temperature of about 175 260 C. which usually detracts from the intercoat adhesion.

The useful amines in my bath composition and replenishment composition therefor are the water soluble amino compounds as described and set forth in Ser. No. 304,297, preferably with 30-60% of the amino equivalents present being contributed by water soluble polyamine. Advantageously, pH of the bath composition is not more than about 8.3 to suppress sorption of CO from ambient air, and generally it will be broadly between about 5 and 8.2 at room temperature. Specific resistance of the bath composition as made up is advantageously between about 700 and 1,000 ohm centimeters to deposit about 25 microns thick as a priming coat, while higher bath resistance gives a thinner film and vice versa. Upon electrodeposition, I estimate that the film should have at least about 40,000,000 ohm-centimeters apparent specific resistance to obtain good throw into interstices and limitation of film thickness to comparatively thin values as electrodeposition proceeds. Advantageously, this apparent specific resistance of film is even higher, preferably at least about 75,000,000 ohm-centimeters for films less than 25 microns thick. Such apparent specific resistance includes a surface resistance factor for the bathdeposited film interface and the film-substrate interface.

The high solids content replenishment composition for sustained operation of the painting bath preferably is, on a pigment and filler free basis, between about 40 and more preferably 50 and 95%, by weight filmforming (straight or extended) resin binder, the upper concentration being so limited to avoid extremely refractory handling properties, and the lower for economy and avoiding the handling of a good deal extra water. While the bath can be replenished with an intimate mixture of pigment and film-forming binder resin, considerable mechanical work is required to achieve the incorporation of that material; accordingly, I have found it advantageous to use up to about 2 parts by weight of water per part of binder resin plus pigment for the replenishment composition.

Preferably, the replenishment composition also con tains a small amount of water soluble amino compound such as a water soluble monoamine, e.g. up to about 10% basis weight of the binder resin and more preferably about l-3% to assist in dispersing the replenishment composition into the bath. Such amino compound in the replenishment composition also assists in hydrating any anhydride groups that are present in conjunction with water in the replenishment composition, thus enhances its dispersibility in the bath. The replenishment composition also can contain for elficiency and convenience, in addition to a controlled proportion of binder resin and pigment, water-soluble amino compound, the small desirable proportions hereinbefore stated of nonionic organic liquid solubilizing assistant and low molecular weight polycarboxylic acid amine scavenger so that the replenishment operates to keep the bath in compositional balance in all respects.

Advantageously, for operating convenience and economy, the proportion of water soluble amino compound entering the bath with the replenishment composition during replenishment operations, either mixed into such replenishment composition or added as a separate and distinct material, is complimentary to the excess Water soluble amino compound that has been built up in the bath from prior electrocoating operations. Together such excess amino compound in the bath and that added to the bath with the replenishment composition maintain the free amino content in the bath at desired level. Accordingly, the amino content of the bath is kept high enough to maintain a stable aqueous dispersion of the synthetic polycarboxylic acid binder resin in the bath as an anionic polyelectrolyte, but not so high as to sorb inordinate quantities of carbon dioxide from the surrounding atmosphere and thus substantially disturb the electrical characteristics of the bath system. Conversely, the synthetic polycarboxylic acid resin binder is serving the auxiliary purpose of being an excess amine scavenger.

Referring to the accompanying schematic drawing depicting one embodiment of apparatus suitable for carrying out the method of this invention, there is shown a steel tank 11 which contains a painting bath 13 and serves as a negative electrode in the coating process. Tank 11 is electrically connected to D.C. power supply unit 17 via conductor 15. An article to be coated 19, e.g. an automobile body, is shown suspended from a conveyor 35 by hangers 21 and 23. Conveyor 35 is a conventional, electrically powdered, chain driven conveyor constructed and arranged for the transportion of articles to be coated through bath 13. Hangers 21 and 23 include insulators 25 and 27, respectively, which insulate article 19 from the grounded conveyor. Contact plates or brushes 29 and 31 are attached to and in electrical connection with hangers 21 and 23, respectively. Article 19 is shown approaching bath 13 and in electrical connection with bus bar 33 which in turn is in electrical connection with D.C. power supply unit 17 via conductor 37. Article 19, therefore, serves as the positive electrode of an electrodeposition cell while the article is passing through bath 13.

It is to be understood that bus bar 33 may be segmented and that certain of the various segments may have no connection with the power supply unit or may be adapted for polarity reversal to provide additional control over the coating process.

Power supply unit 17 is constructed and arranged to provide between the electrodes and through the coating bath a direct current flow of electrical energy that is commensurate with the size of the electrocoating operation contemplated. Design of the power supply unit should take into consideration the surface area of the workpieces that will be in residence within the coating bath at any given time, the workpiece surface area entering the bath per unit time, the deposition properties of the coating formulation, the conductivity of the coating bath, the thickness of the coating to be formed, etc. Ordinarily, such current will be provided by rectification of an alternating current power source by conventional means.

The following examples show ways in which my invention can be practiced, but should not be construed as limiting the invention. The electrical equivalent weight of the polycarboxylic acid resin binders are between 1,000 and 2,000. All parts are parts by weight and all percentages are weight percentages in this application unless otherwise expressly indicated.

EXAMPLE 1 A paint binder is made by reacting in an agitator tank 8,467 parts of alkali-refined linseed oil and 2,025 parts of maleic anhydride (heated together at 232.2 C. for about 3 hours until an acid value of 8090 results), then cooling this intermediate to 157.2 C., adding 1,789 parts of vinyl toluene containing 48 parts of ditertiary butyl peroxide and reacting at 216 C. for about an hour. It has acid number of 75-85 at this stage. The resulting vinyl toluenated material is then cooled to 190 C. and 2,647 parts of non-heat reactive, thermoplastic, oilsoluble phenolic resin are added, the temperature goes to 177 C. and the mixture held 1 hour to unite the acid resin and extender, then cooled to about 150 C. and strained. The phenolic resin is a solid lump resin having softening point of 120-140 C., specific gravity of 1.03 1.05 at 20 C., and has been stripped to get out excess phenol and low molecular weight materials. It is a condensation product of about equimolar quantities of para tertiary butyl phenol and formaldehyde. The electrical equivalent weight of the resulting acid resin as extended is about 1,400.

A thick blend is then made by mixing together 2,400 parts of the foregoing extended acid resin with 211 parts of water, and 29 parts of triethylamine, then adding and mixing in 25 parts of mineral spirits, a light hydrocarbon liquid having API gravity of 45-495, specific gravity at 15.6 C., of 0.78-0.80, flash point (Cleveland Open Cup) between 37.8 and 46 C., a negative Doctor test and no acidity, 50 parts of 20 butoxyethanol, 100 parts of pine oil, 15 parts of antifoam agent (a ditertiary acetylenic glycol with methyl and isopropyl substitution on the tertiary carbon atoms), and 1,615 parts of pigment "rind.

a The pigment grind is made of 497.1 parts of fine kaolin clay having average particle size of 0.55 micron with particle size distribution as follows: 95% less than 2 microns, 50% less than 0.55 micron, 20% less than 0.35 micron; 243 parts of pigmentary titanium dioxide having average particle size about 0.3 micron and substantially no particles as large as a micron; 166 parts of fine lead chromate having average particle size of 0.10.5 micron with less than 25% of the particles exceeding 44 microns; 146.4 parts of fine red iron oxide having average particle size of 0.55-0.6 micron with less than .5% of the particles exceeding 20 microns; 2.8 parts of carbon black having average particle size of about 25 millimicrons with less than 0.2% particles exceeding about 44 microns; 262.5 parts of a mixture containing 104.8 parts of a vinyl toluenated, maleic couple linseed oil made in the same manner as the resin hereinbefore shown in this example (except that the resulting polycarboxylic acid resin is not extended with any phenolic resin), 25 parts of diisopropanol amine, and 132.7 parts of water; 408.9 parts of additional water; 1.7 parts of the acetylenic glycol antifoam agent referred to above; and a mixture of 18 parts of diisopropanol amine and 18 parts of Water.

The resulting thick blend, 4,445 parts, is mixed with 3,517 parts of water and 500 parts of a neutralizing solution consisting of 12.4% diethylene triamine, 43.8% dirsopropanol amine, and 43.8% water to yield 8,462 parts of heavy paint having about 40.7% total solids (N.V.M. after 20 minutes at 176.7 C.). To this body of heavy paint 1s added 29,804 parts of water to yield a bath dispersion of 38,266 parts having 9% total solids by weight. For practical estimation purposes, the non-volatile matter can be taken as the resins plus pigments, and all else can be considered volatile.

The cathode is an agitated metal tank holding 1,000 parts of the bath composition at 9% solids and surrounding the anode with about 2.5 cm. clearance at the sides and bottom. The anode is a 0.102 meter x 0.102 meter square of 24 gauge sheet steel. Voltage across anode to cathode is constant at 350. The anode linear insertion rate 1s 0.3 meter per minute, and the anode immersed to a depth of .089 meter. In the painting operation, the tank temperature is controlled between 21 and 35 C. The specific resistance of the bath initially is 750-],050 ohmcentimeters.

The resulting coated anode is removed from the bath with power on after 60 seconds have elapsed. The anode coating has resin-to-pigment ratio of about 2.1 1, whereas the original bath dispersion has resin-to-pigment ratio of 28:1 and is of suflicient volume to remain substantially constant, although decreasing very slightly, during the 1-minute run.

The anode is baked for 15 minutes at 176.7 C. The coating before baking has no visible surface irregularities, and the film is slightly tacky. Adhering droplets are blown off with air. After baking, the film exhibits excellent adhesion and is pore-free and durable, exhibiting standard salt spray resistance in excess of 250 hours. The cured paint film is smooth, even, of uniform sheen, and about 25 microns thick.

For sustaining this operation, a high painting solids content (74.5%) replenishment composition is made like the foregoing thick blend except that 2,185 parts of the pigment grind is used, the weight ratio of binder resin-topigment being 2.1 1.

A succession of panels are electrocoated like the first one. For every 10 parts of paint solids deposited on the panels, about 13.4 parts of the replenishment composition and 0.275 part of aqueous amine solution (12.4% diethylene triamine, 43.8% diisopropanol amine, and 43.8% water) are stirred into the bath to maintain both solids content between 8 and 9%, with a small amount of additional water being added periodically to maintain bath volume essentially constant for evaporation and entrainment losses. The clectropainting and curing operations yield a series of panels essentially of the same good quality as the first one and substantially consistently having a binder resin-to-pigment weight ratio of about 2111, while the binder resin-to-pigment Weight ratio in the bath composition is thus maintained at about 2.8:1.

EXAMPLE 2 A steel panel is coated essentially in the manner of the first one in Example 1 except that the component pigments are a kaolin clay having average particle size of 9.5 microns with particle size distribution about as follows: .5 larger than 44 microns, 50% smaller than 10 microns, and 10% smaller than 3 microns; the same kinds of pigmentary titanium dioxide, lead chromate, and carbon black as in Example 1; and red iron oxide having average particle size of about 2 microns with 1% larger than 44 microns. The resulting cured panel is roughened in contrast to the smooth uniform film of Example 1, and it has areas of varying sheen across the panel. Furthermore, when the film is examined from top to bottom with magnification, there is a pronounced binder of enrichment at the base of the film in contact with the anode. The filmsurface is distinctly inferior to that of the panels of Example 1.

EXAMPLE 3 A steel panel is coated essentially in the manner of the first one in Example 1, except that the resin-to-pigment ratio in the initial operating bath is 2:1 instead of 2.821. The resulting cured paint film on the panel is substantially less glossy than the panels of Example 1, but the film is still serviceable and not chalky. However, when the same operation is performed using a resin-to-pigment ratio of 1.511 in the bath, the resulting resin-to-pigment ratio deposited on the anode and cured is about 1.121. This film is pronouncedly chalky and weakened, and there is noticeable pigment enrichment at the top of the film instead of substantial homogeneity.

EXAMPLE 4 A steel panel is coated essentially in the manner of the first one in Example 1 except that there is added to the thick blend made with the 2,400 parts of extended acid resin 288 parts of a dimer acid (the polymerization product of C unsaturated aliphatic monobasic acids, principally linoleic), having about 83% of a C dibasic acid of about 565 molecular weight and about 1% max imum of C fatty acids having molecular weight about 282, the dimer acid having acid number of 188-196, saponification number of 192-198, a color on the Gardner scale of 8, and a neutralization equivalent of 287289.

The resulting electrocoated panel is baked for 20 minutes at 176.7 C. When this cured prime coat is sprayed with a melamine-modified enamel top coat and the top coat cured to dryness by baking, there is improved intercoat adhesion between the electrocoated film and the topcoating film on the finished panel in comparison to the same sort of panel operation wherein no dimer acid 1s used in the primer. Similarly, when the baking temperature of an electrocoated panel is raised to about 200 C., the panel electrocoated with such primer containing the dimer acid, sprayed with the same melamine-modified enamel topcoat, and finally cured has still good intercoat adhesion in spite of the high curing temperature used on the primer. It is substantially better than the mtercoat adhesion obtained when no dimer acid is used in the primer coat.

EXAMPLE 5 A thick blend is made of 91 parts of extended acid resin like that of Example 1, 7.9 parts of triethylamme, 1.1 parts of water and 25 parts of pigment grind. The pigment grind is made in the proportion of: 226 parts of fine kaolin clay; 21 parts of fine lead chromate; 194 parts of a mixture containing 176.5 parts of a vinyl toluenated maleic coupled linseed oil made in the same manner as the resin in Example 1, but not extended with any phenolic resin; 2.1 parts of water; and 15.4 parts of triethylamme; 1 part of the acetylenic glycol antifoam agent described in Example 1; and 93 parts of Water. The lead chromate and clay are the same kinds as used in Example 1.

The thick blend is mixed at about 66 C. With 18 parts of a neutralizing solution consisting of 12.4% diethylamine triamine, 43.8% diisopropanol amine, and 43.8% water. After about 15 minutes of mixing, sufiicient Water is added to reduce the concentration of painting solids to about 36% To make up the painting bath, the 36% dispersion is cut to 8% painting solids content with Water. The resinto-pigment ratio of the painting bath is 10.5 :1. On electrodepositing and baking, this material like the first panel in Example 1, a virtually clear-looking film is made. It has resin-to-pigment ratio of approximately 8:1 and exhibits salt spray resistance of hours in a standard salt spray test. When the same kind of resin without any pigmentation is deposited in the same manner, the standard salt spray resistance is only about 30 hours.

EXAMPLE 6 An alkyd resin is made by heating 948 parts of tall oil fatty acids containing 97.6% tall oil fatty acids, 1.2% rosin acids, and 1.2% unsapon-ifiable content, having acid number of 197, Sap. No. of 198, and an iodine value of 128, and parts of maleic anhydride at 232 C. for 1 hour, cooling the mixture to 104.4" (3., adding 503 parts of technical grade pentaerythritol, 394 parts of phthalic anhydride, and 30 parts of xylol (for water entraining solvent which is removed from the resin after the batch is made), then refluxing the batch at 171 C. with water of reaction separation until an acid number of 102.6 is reached on the resulting resin.

A pigment grind is made using 104.8 parts of the foregoing alkyd resin, 132.7 parts of Water, 25 parts of diisopropanolamine, 497 parts of fine kaolin clay. 243 parts of pigmentary titanium dioxide, and 16.6 parts of lead chromate, the pigments being the same kind as used in Example 1.

84 parts of pigment grind are :blended with 125 parts of the alkyd resin, 10 parts of Water, and 1.3 parts of triethylamine to make a thick paint dispersion. This dispersion is thereafter mixed with 31.8 parts of diisopropanolamine, 5.5 parts of diethyltriamine, and suificient water to reduce the resin solids concentration to 5%.

The painting operation is conducted on a panel like the first panel coated in Example 1 from a bath having 2.8:1 resin-to-pigment ratio. On baking for 20 minutes at 176.7 C., the film cures to give an excellent pore-free, durable coating that is substantially homogeneous and has a resin-to-pigment ratio of about 2.1:1. Lowering the resin-to-pigment ratio in the bath below about 2:1 gives similar effects to those shown 11 Example 3.

EXAMPLE 7 An acrylic resin is made by slowly adding a mixture of 60 parts of butyl acrylate, 25 parts of styrene, 15 parts of methacrylic acid, 1 part of t-butyl perbenzoate, and 1 part of benzoyl peroxide into 34.7 parts of 2-butoxy ethanol maintained at 157.2- C. during a 2 and /2 hour period and is held for an additional hour at this temperature using an agitated reactor equipped with a reflux condenser. The resulting resin is cooled to l37.8 C. and further reacted for /2 hour at 154.4 C. With 10 parts of tris hydroxyl methyl amino methane to enhance its apparent water solubility. The resulting resin dispersion has acid number of 57.6 and 75.6% resin solids content and is apparently soluble in the painting bath hereinafter described.

A White paint concentrate is made up by blending 133 parts of the foregoing resin dispersion, 10 parts of a water soluble amino compound mixture (composed of 86 parts diisopropanol amine, 25 parts diethylene triarnine, and 86 parts water), 30 parts of a pigment grind (composed of 16 parts of titanium dioxide, 8 parts of fine kaolin clay, and 6 parts of the alkyd resin shown in Example 6 and 47 parts of water).

The bath for electrocoating is prepared by stirring the White paint concentrate with 10 additional parts of the amino compound mixture (86 parts diisopropyl amine, 25 parts diethylene triamine, and 86 parts Water) and 780 additional parts of Water to make up an agitated liter bath having 12.8% resin solids and pH of about 7.5. The resinto-pigment ratio of the bath dispersion is 10.7: 1. A panel is electrocoated like the first one in Example 1. Ovendrying of the coated panel for 20 minutes at 176.7 C.

gives a durable glossy film of high quality having a resinto-pigment ratio of approximately 8.1. When the resin-topigment ratio in the bath is made up to be below about 2: 1, the paint film shows deficiencies similar to that of the corresponding'paint film in Example 3.

EXAMPLE 8 Painting baths like that of Example 1 are made up, except that the concentration is varied in each by regulating their dilution with water. An anode is electrocoated and baked in the manner of the first one in Example 1 from each bath concentration. The following table sum marizes the results.

Total solids concentration Viscosity centipoises in the bath (NV'M): at 267 C.

2 Virtually the same as Water.

At 2% painting solids, the panel shows some orange peeling and streamline deposits are beginning to appear in streaks or ridges. At 37.5% solids, the painting solids losses are high from the bath by viscous adhesion of paint to the anode as it is removed from the bath. At the two intermediate concentrations, such losses are much reduced.

EXAMPLE 9 500 parts of the acrylic resin shown in Example 7 are blended with 94.2 parts of a 50% by weight diisopropanolamine-water solution and further reduced with water to give a 40% resin solids dispersion.

A pigment grind is made up by grinding together 79 parts of this reduced acrylic resin dispersion, 76 parts of titanium dioxide, 18.7 parts of fine kaolin clay, and 78.3 parts of water, the titanium dioxide and clay being the same kinds as used in Example 1.

A second mixture is ground together; it has 106 parts of very finely-powdered polyvinyl chloride-vinyl acetate copolymer resin having melting point about 150 C., 89.7 parts of water, and 84.3 parts of the reduced acrylic resin dispersion.

A composition for electrodeposition is made by blending together 106 parts of the pigment grind, 75.8 parts of the copolymer resin grind, 134.5 parts of the 40% solids acrylic resin dispersion, and 693.7 parts of water. It has acrylic resin-to-mineral pigment ratio of about 1.9:1 and acrylic resin to particulate solids (that in mineral pigment plus polyvinyl chloride resin) of about 1.1: l. The electrodeposition is done like that of Example 1.

When such electrocoated anode is cured at room temperature, the film is poor and chalky because the filmforming binder resin-to-pigment ratio is about 0.87:1, with the hard copolymer resin effectively acting as a component pigment in such operation. However, when the electrocoated anode is cured by baking at 176.7 C. for 20 minutes, the film smoothes out to a good, serviceable film that is not chalky; it has an acrylic resin-toinfusible mineral pigment ratio 15:1 and this baked film is further enriched by the auxiliary binder of the hard copolymer resin to yield an overall resin-to-pigment ratio of about 2.2: 1.

Similarly, good appearing films were electrodeposited on panels and baked using, for example, as the supplementary resin filming adjunct fusible on baking very finely-divided (a) chlorinated hydrocarbon wax; (b) epichlorohydrin-bisphenol A type epoxy resin having molecular Weight of about 1,000; (c) chlorinated rubber; and (d) rosin-modified phenol-formaldehyde resin. Of these, Resin (b) is found to have cross-linked with the acid binder resin upon baking. In film thus cured, such filming adjuncts are not pigmentation for my painting or replenishment compositions in the sense used in this application, but they contribute to the total painting solids of the painting composiiton because they remain as nonvolatile matter after baking in air for 20 minutes at 176.7 C.

As aforementioned and illustrated, the essentially nonionic, particulate, filming adjunct resins deposit from the bath in substantially the same manner as mineral pigment but if fusible at the temperature of baking become a part of the binder film either through chemical cross-linking or physical blending. In embodiments wherein such adjuncts are employed, control of binder-to-pigment ratio in the film is provided by maintaining an ionic binder resin-to-pigment weight ratio in the bath of at least about 1.5: 1, preferably at least about 1.7: l, and employing the fusible, particulate, filming adjunct in an amount sufficient to raise the total resin-to-pigment binder to at least about 2:1.

Since the particulate filming adjunct resin and mineral pigment deposits at substantially the same rate, maintenance of film composition may be controlled by employing a replenishment feed in which the Weight ratio of filming adjunct resin-to-mineral pigment is substantially equal to the weight ratio of film adjunct resin-to-mineral pigment in the bath and the weight ratio of ionic resin binder-to-mineral pigment is at least about 1.5: 1.

It will be understood by those skilled in the art that modifications can be made in the details of the examples hereinbefore set forth without departing from the spirit and scope of the invention as expressed in the claims.

What is claimed is:

1. A multi-component painting composition for use in elecrto-deposition of a paint film on an anode immersed therein when a direct electric current is passed through said composition between said anode and a spaced apart cathode in contact with said composition, said composition comprsing a fluent, intimate, mixture of (a) water (b) pigmentation consisting of at least one component pigment, said pigment having an average particle size not substantially above about 5 microns with no more than 5% thereof being larger than about 10 microns,

(c) film-forming binder resin for pigment, at least a major fraction of said binder resin being synthetic polycarboxylic acid resin, and

(d) sufficient water soluble amino compound to maintain said polycarboxylic acid as a dispersion of anionic electrolyte in said composition, wherein the total painting solids comprise about 2% to about 15% by weight of said bath and the remainder consists essentially of Water and water soluble amino compounds and the weight ratio of said binder resin to said component pigment in said composition being at least about 2:1 and substantially above the binder resin-to-pigment ratio in the paint film that will electrodeposit from said composition upon said anode when passage of said current is initiated.

2. The composition of claim 1 wherein said pigment has an average particle size not substantially above about 2 microns when said pigment has a specific gravity above about 3.5, and said pigment having an average particle size not in excess of about 5 microns when said pigment has a specific gravity below about 3.5.

3. A replenishment composition for replenishing the paintaing composition of claim 1, said replenishment composition comprising:

(a) pigmentation consisting of at least one component pigment, said pigmentation having average particle size not substantially above about 5 microns, with no more than about 5% thereof being larger than about 10 microns;

(b) film-forming binder resin for pigment, at least a major fraction of said binder resin being synthetic polycarboxylic acid resin;

(c) the ratio of said binder resin to pigment being at least about 1.5 :1 and substantially the same as the ratio of said binder resin to said pigment in the paint film that will electrodeposit from said bath;

(d) to 2 par-ts by weight Water per part of said binder resin plus pigment; and

(e) about 0 to about 10% water soluble amino compound basis weight of said binder resin, the portion of said amino compound being insufficient for inducing stable aqueous dispersion of said polycarboxylic acid resin as an anionic polyelectrolyte in said composition.

4. A high solids content replenishment composition for dispersion into an aqueous electrocoating paint composition bath adapted for the electrodeposition of a paint film therefrom on an anode immersed therein when a direct electric current is passed through said bath between said anode and a spaced apart cathode in contact with said bath, said replenishment composition comprising an intimate mixture of (a) pigmentation consisting of at least one component pigment,

(b) about 40% to 95% by Weight film-forming binder resin, on a pigment and filler-free basis, and

(c) about 0.1 to about 10% Water soluble amino compound basis Weight of said binder resin, at least a major fraction of said binder resin being synthetic polycarboxylic acid resin having acid number between about 30 and about 300, the proportion of said amino compound being insufiicient for inducing stable aqueous dispersion of said polycarboxylic acid resin as an anionic polyelectrolyte in water, the weight ratio of said binder resin to said pigment in said replenishment composition being at least about 1.5 :1 and substantially the same as the ratio of said film-formbinder resin to said pigment in the paint film that will electrodeposit from said bath when passing of said current is initiated.

5. The composition of claim 4 wherein said pigment has an average particle size not substantially above about 5 microns, with no more than 5% thereof being larger than about microns.

6. The composition of claim 4 wherein said poly- 16 carboxylic acid resin has electrical equivalent weight between about l,000 and about 20,000.

7. In the method for preparing a high solids content replenishment composition for dispersion into an aqueous paint composition used for a direct current electrodeposition painting operation, wherein the binder resin to pigment ratio of said paint composition is at a walue R and the binder resin to pigment ratio in an anodic film electrodeposited from said direct current electrodeposition of R wherein R is at least about 1.521 and substantially below said value R the improvement which comprises, formulating an intimate mixture comprising pigmentation consisting of at least one component pigment, film-forming binder resin about 0.1 to about 10% of water soluble amino compound basis weight of said binder resin, and 0 to 2 parts of water per part of said binder resin, at least a major fraction of said binder resins being synthetic polycarboXy-lic acid resins having an acid number between about 30 and about 300, the proportion of said amino compound being insuflicient for inducing stable aqueous dispersion of said polycarboxylic acid resin or an anionic polyelectrolyte in water, wherein the binder resin to pigment ratio in said intimate mixture is substantially equal to said value R References Cited UNITED STATES PATENTS 3,448,027 6/1969 Hart et al. 204-181 FOREIGN PATENTS 633,074 10/1963 Belgium 260l8CL OTHER REFERENCES Paint Technology, Electrodeposition of Paint-3, J. Berry; vol. 28, No. 3, March 1964, pp. 53 to 58.

DONALD E. CZAIA, Primary Examiner C. W. IVY, Assistant Examiner US. Cl. X.-R.

204-l8l; 260-l8, 19, 23, 25, 29.3, 29.6, 29.7, 37

Disclaimer 3,575,909.Allan E. Gilchrist, \Vestlake, Ohio. ELECTRODEPOSITION BATH COMPOSITION AND REPLENISHMENT COMPOSI- TION THEREFOR. Patent dated Apr. 20, 1971. Disclaimer filed July 26, 1971, by the assignee, Ford Motor Company.

Hereby disclaims the portion of the term of the patent subsequent to J an. 9, 1985.

[Ofioz'al Gazette October 5, 1.971.] 

